1. Field of the Invention
This invention relates to zoom lenses, and more particularly to focusing methods and means that move one of the zoom components.
2. Description of the Prior Art
In general, many zoom lenses are constructed as illustrated in FIG. 1. Such zoom lenses include, from front to rear, a focusing component 12, a zoom assembly composed of two components 13 and 14, and a fixed component 15. Methods and means of focusing that involve moving the front or first component, i.e. front focusing, offer the advantage that for a given object distance, the focusing component moves a constant distance regardless of the zoom setting of the lens. Allowing for forward movement of the front component when focusing down to shorter object distances, however, requires making the diameter of the front component large. This is a disadvantage.
Previous proposals for decreasing the diameter of the front component suggested focusing with other components other than the front components, for example either with one or both of the zoom components, or using the so-called inner focusing method. One of these proposals, as for example disclosed in U.S. Pat. Nos. 3,391,973 and 4,364,642 is that schematically illustrated in FIG. 2. Here the zoom lens includes a fixed component 22, two zoom components 23 and 24, and a fixed component 25 with a focal plane at 26. This device is focused with either one of the components 23 and 24 or with both of them and comprehends a variation of the total focusing movement during zooming. In general, the longer the focal length of the entire system, the larger the total focusing movement which is required to effect an equivalent result. Therefore, after the zoom lens has focused on a finite object, zooming shifts the position of the image of sharpest focus. In every such event, therefore, the photographer needs to set the focusing actuator so as to correct for the image shift.
Accordingly, conventional rules of design of zoom lenses, though advantageous at minimizing the size of the front component, raise an alternative problem, namely that even for one and the same object distance, the required movement of the focusing component necessarily differs with the zooming position.
Another proposal, that disclosed in Japanese Laid-Open Patent Application Nos. SHO 56-165106 and SHO 56-165107, is that of focusing with one or all of the components that follow the zoom components. This narrows the range of variation of the total focusing movement when zooming.